Objective-C concurrency issues

Disclaimer - I've not shipped Java Swing / SWT apps. I'm a server guy where markup is the UI. Consequently, I don't have in-depth knowledge of Java to compare against. I'm aware of SwingUtilities.invokeLater(Runnable) but otherwise just assume I'm clueless about Swing.

First rule of GUI programming - don't block the main thread.

Second rule of GUI programming - don't block the main thread, etc.

Quantifying this, you have a device running at a refresh rate of 60Hz. So you if you do anything in the main thread, you need it to complete in under 16ms, or your UI will not be smooth and responsive.

In Java, I would normally look at Executors, Callables, Runnables and related APIs to do things off the main thread. In Objective-C, we have NSOperationQueue and NSOperation. Learn, use and love them. In particular, don't do what I did and start porting java.util.concurrent classes to Objective-C. I wrote a CountdownLatch, which was very nice and taught me about various low-level concurrency primitives. Unfortunately it was completely the wrong solution for the language. What I should have done was to use [NSOperation addDependency:] to chain tasks together.

Objective-C tooling

Java development tools are top of the pile out of anything I've used. The IDEs are massively powerful (they have to be, with the warts on the language). I'm also an emacs user day to day, and pragmatically use vim as well. But Eclipse / IDEA / Netbeans are pretty amazing tools for Java The Language development.

Respectively for Objective-C development, Xcode isn't.

If Apple Ts&Cs allow, IntelliJ could probably make some impressive inroads into that market.

clang is a good (and getting better all the time) addition. The debugger needs some love; I don't find gdb as powerful as Java debuggers.

In Java-land, one can use maven, ant, ivy, Make, etc to build a project. For iOS development, the IDE rules a lot from the off. There is a command-line tool which can potentially be driven by Jenkins or Thoughtworks Go. That would be my preferred option going forward; in my view, building in an IDE is not a repeatable build process.

Apple AppStore feedback

This morning the AppStore on my iPod Touch pointed out that the Twitter app had an update. I installed it and launched the app to see what was new. Crash! After 5 crashes in a row without a successful launch, it was obvious that a bad build had snuck into the App Store.

Incident Identifier: 61EE3335-1AD0-4099-8EC6-FAB4B6160A43

CrashReporter Key: 001e29bd4aa2ef81d42701ce5325da94b364e27b

Process: Twitter [8470]

Path: /var/mobile/Applications/8E13E345-CDD3-4CA4-899D-8E38BA6661C5/Twitter.app/Twitter

Identifier: Twitter

Version: ??? (???)

Code Type: ARM (Native)

Parent Process: launchd [1]

Date/Time: 2010-08-04 07:58:35.994 +0100

OS Version: iPhone OS 3.1.3 (7E18)

Report Version: 104

Exception Type: EXC_BREAKPOINT (SIGTRAP)

Exception Codes: 0x00000001, 0xe7ffdefe

Crashed Thread: 0

Dyld Error Message:

Symbol not found: __NSConcreteGlobalBlock

Referenced from: /var/mobile/Applications/8E13E345-CDD3-4CA4-899D-8E38BA6661C5/Twitter.app/Twitter

Expected in: /usr/lib/libSystem.B.dylib

Dyld Version: 149

Binary Images:

0x1000 - 0x14ffff +Twitter armv6 <43ca857e309a61ba8c5da3ab83e42218> /var/mobile/Applications/8E13E345-CDD3-4CA4-899D-8E38BA6661C5/Twitter.app/Twitter

As an iPhone app developer, I think I know what this problem is. We saw this problem in one of our apps. IIRC, the new, preferred llvm compiler has a bug with the new blocks language construct, and gcc doesn't, and the bug only shows up at runtime, in certain environments. To fix it, Twitter are going to have to recompile and use gcc rather than llvm, and then wait for the wheels at Apple to turn.

Other people have talked about the frustration of not being able to iterate at web speed or do continuous deployment, but that's part of the ecosystem that you operate in with Apple.

Testing, either by the Twitter team, or by Apple when they review the app prior to approving it, should have caught this issue. But these things happen.

We had a similar thing happen with an update to one of our apps recently. An update went live and thanks to the apparent difficulty in doing your own testing of the binary that gets sent to Apple, an issue only became apparent when the new version was available through iTunes. To me, this is where the ecosystem is broken. If I have a webapp and I deploy an update, then find an issue (via my cluster-immune system - one day!), I roll it back.

iTunesConnect has no rollback, even though it seems like a highly desirable feature. I know in our case, we would have liked the option to rollback to the last known good version and then wait for Apple to review an update, rather than having the world upgrade to a version that we didn't want them to be running. I imagine Twitter would appreciate a similar feature right about now.

Capturing Mobile Network Traffic On OS X

Recently had to audit an app to ensure that it wasn't leaking any unwanted details over the network. This was an iPhone app, but the same process can be used for Android, etc.

1. Ensure Macbook Pro is plugged into Ethernet.
2. Open System Preferences
3. Internet & Wireless | Sharing (in Snow Leopard).
4. Click Internet Sharing.
5. From Ethernet
6. On Airport
7. Close System Preferences
8. Click Airport
9. Select Create Network...
10. On the phone, open the WiFi controls and connect to the network that you've just created.
11. Run Wireshark.
12. Start capturing traffic on the wireless card.
13. Check stuff is using SSL that should be, etc.

Objective-C - the language

First off, I read the Objective-C Primer and Objective-C Programming Language guides. I collect languages, so there was some underlying familiarity there. Ruby, Smalltalk and C obviously shone through for me. Second off, I re-read Smalltalk Best Practice Patterns. I first read that book maybe 6 years ago and it had a massive impact on my Java style. Objective-C is the most Smalltalk-like language that the 'masses' will actually use professionally. Sadly, it's not enough Smalltalk for me, and the C abstractions leak quite a bit.

Java Developers Guide to Objective-C on the iPhone

This will be a place-holder page containing links to the other entries that I create in this series. I've got a lot of commercial experience with Java, some Python and Ruby. This has all been server-side; I've not really touched GUIs (apart from GWT, HTML and Javascript) for a while, so this series will necessarily reflect that. Hopefully it will prove useful to others.

Topics that I hope to cover:

• Initial questions coming from a mainly Java background.
• Objective-C the language, including comparisons with Java.
• OO with Objective-C, covering how a typical Java app would use interfaces and how Objective-C might approach the problem.
• Collections in Java and Objective-C.
  
• Concurrency utils in Java and Objective-C.
  
• Tools.

Objective-C for Java Developers

I'm coming from an Eclipse on Ubuntu background, but this is equally applicable for IDEA on Windows. What are the equivalents for iPhone development?

Java	iPhone	Notes
JUnit (unit testing framework)	?	It is possible to use TDD for Swing apps, although I've been predominantly a server-side guy with client stuff happening in the browser for quite a while now. Cucumber with iPhone looks worth exploring...
Hudson (continuous integration tool)	?	On my first iPhone app, it rapidly became apparent how easy it was for people to do bad merges and delete classes from the Xcode project file / strings from the UTF-16 l14n Localizable.strings file. You can argue that people should take more care; yeah, that'll fix it. git bisect is great, but a tool that builds on each commit is better.